Any suggestions, please - Truma Combi water heater

 .... I assume you've opened all the taps in the caravan 👍

On ours there is an elbow where the hot water leaves the boiler at the top between the copper pipe from the boiler and the flexible pipe that carries it to the taps. That has a valve in it connected to a pipe that exits the floor of the vehicle. As far as I know that let’s air in as the boiler drains and seals when the boiler is filled. I had to replace ours as it was not re sealing when the boiler was full and water kept running out of the pipe through the floor. Perhaps in your case rather than sticking open, it is sticking closed. It was a fairly easy job to replace.

Yes, all taps open in central position. I took off the shower head and blew down the hose. This expelled about another half cup full of water.

I've noticed the same thing with our Combi 6E although it's not impacted on nose weight as far as I know.  I always aim to run at about 90-95kg (and check it every time we set off) and that hasn't changed (other than when Ros excels herself with the amount of clothes she 'has to take - just in case'

There are 4 drain valves in the Knaus - the frost protector and hot water drain near the boiler plus a hot and cold drain at the rear of the van.  I open all when draining down.

Sometimes - and it happened this week after an hour's long tow - it takes very little water to fill the boiler - but other times it takes an age to bleed the system.  There doesn't seem to be any consistency to it.

I've just ignored it since I always have some heating on in the van in potentially frosty weather when the van's at home and with that and the frost protector valve, I'm not unduly concerned.  Does baffle me, though.

I'd also noticed how quickly water was coming through the taps. This should have alerted me to a possible problem. Usually it takes 2 or 3 minutes for the spluttering to stop.

We had a similar problem about 20 years ago, it was to do with the blue/yellow drain valve that you pull vertical to drain the water. The inside was faulty somehow and would not let any air in to let the water come out. It was under warranty and I took it to the dealer and they knew almost immediately what was wrong and once replaced everything worked again. I watched them replace it and it took under five minutes and very easy as it was all 'pull the collar' to get the pipes out then 'push fit', in fact I still carry a spare (somewhere in the front locker) with me in case it happens again. Perhaps contact a mobile engineer? 

My mobile service engineer has just got back to me with what he is almost certain is the cause of the problem. At the top of the boiler is a non-return valve, as already described above by SteveL, (thanks Steve)  "A common problem" says my engineer. Very easy and quick to replace for less than £20 so hopefully problem solved but can't order one until Tuesday. Who'd have bank holidays, they shouldn't be allowed !!!

Removed the non return aeration valve (push fit) and the residual water of about 7 litres drained out under the caravan as it should. Pushed the valve back onto the boiler until I can get a new one, hopefully this coming week. Whole exercise took less than 10 minutes. 

Hope things remain simple and inexpensive John👍